Science Earth`s Changing Surface

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Transcript Science Earth`s Changing Surface

Earth’s Changing
Chapter 4
Section 2
Finding the Relative Age of Rocks
Relative means compared to others
The relative age of a rock does not provide
its absolute age
The absolute age of a rock is the # of yrs
since the rock formed
The Position of Rock Layers
The Law of Superposition is used to
determine the relative ages of sedimentary
rock layers
In horizontal sed rock layers, the oldest layer is
@ the bottom
Each higher layer is younger than the layers
below it.
Other Clues to Relative Age
Extrusion is lava that hardens on the
Intrusion is where the magma cools &
hardens into a mass of igneous rock
The rock layers below an e are always older
The i is always younger than the rock layers
around or beneath it
A fault is a break in E’s crust
Gaps in the Geological Record
The surface where new rock layers meet a
much older rock surface beneath them is
called unconformity
This is a gap in the geological record
Using Fossils to Date Rocks
Index fossils help geologists match rock
To be useful as an index, a ‘f’ must be
widely distributed & rep a type of org that
existed only briefly
I.F. are useful because they tell the relative
ages of the rock layers in which they occur